In a carriage printer, such as an inkjet carriage printer, a printhead is mounted in a carriage that is moved back and forth across the region of printing. To print an image on a sheet of paper or other print medium, the medium is advanced a given nominal distance along a media advance direction and then stopped. While the medium is stopped and supported on a platen, the printhead carriage is moved in a direction that is substantially perpendicular to the media advance direction as marks are controllably made by marking elements on the medium—for example by ejecting drops from an inkjet printhead. After the carriage has printed a swath of the image while traversing the print medium, the medium is advanced, the carriage direction of motion is reversed, and the image is formed swath by swath.
FIG. 1 shows a schematic side view of a prior art carriage printer having a so-called L-shaped paper path. A variety of rollers are used to advance the medium through the printer. In this example, a pick roller 350 moves the first piece or sheet 371 of a stack 370 of paper (also generically called recording medium herein) at media input support 320 from paper load entry direction 301 toward media retention plate 340. Media retention plate 340 is disposed along media advance direction 304 and is at an angle α with respect to media input support 320. Angle α is typically greater than 60 degrees, so that when seen from the side view of FIG. 1, media input support 320 and media retention plate 340 look approximately like a letter L. The piece 371 of recording medium is then moved by feed roller 312 and idler roller(s) 323 to advance through the print region 303, and from there to a discharge roller 324 and star wheel(s) 325. Carriage 200 moves a printhead die 251 along a carriage scan direction that is into the plane of FIG. 1 and ink drops 270 are controllably ejected to print an image as the carriage is moved. Supporting the piece 371 of recording medium at print region 303 is a platen 390. In order to facilitate the printing of borderless prints where the image is printed to the edges of the recording medium, platen 390 can have support ribs 394 in between which is disposed an absorbent medium 392 to catch ink drops that are oversprayed beyond the edges of the recording medium.
In order to provide sufficient frictional force to advance top piece 371 of recording medium from the stack 370 during the pick operation, pick roller 350 is provided with a high friction surface and is typically biased against the stack 370 with a biasing force. The biasing force can be provided, for example by a spring, or by the weight of the pick roller assembly, or by the motor that drives the rotation of the pick roller. Although the biasing force facilitates the pick operation, the biasing force must be overcome in order to load additional pieces of recording medium at the media input support 320. If the recording medium is sufficiently stiff, or if there is a sufficient number of sheets of recording medium to provide overall stiffness of the stack being loaded, the recording medium itself can push the pick roller away while the recording medium is being loaded. However, when only a few sheets of low stiffness are being loaded, the biasing force on the pick roller can result in wrinkled or damaged recording medium, which can lead to paper jams in some instances. Since a user may wish to load only a few pieces (or even a single sheet) of low stiffness recording medium, a way of reliable paper loading must be provided for such instances.
In a paper feeding apparatus described in U.S. Pat. No. 6,547,235 the “draw-out roller” (similar in function to a pick roller) can be moved away from the stack or media or toward the stack of media by using the same motor that is also used to cause the pick roller to rotate. However, that paper feeding apparatus uses two motors and two one-way clutches, requiring additional space and cost.
What is needed is simple, low cost and compact way of overcoming the pick roller's biasing force so that even one or two sheets of low stiffness recording medium can be reliably loaded into the printer without causing wrinkling or other damage to the recording medium.